Centrifugal fans are inherently noisy machines, due to the design and airflow interaction of the fan wheel and blower outlet. Air is drawn in at an inlet by a motor-driven rotating impeller. The impeller includes a number of passages arranged in a spiral. Air accelerates through these passages and emerges at an outlet. A cut-off area between the impeller housing and the outlet causes a sudden change of radial and tangential airflow at the outlet. The change in airflow, which is proportional to the blower speed, causes a pressure pulse that results in noise generation.
Conventional efforts to reduce noise generated by centrifugal fans include insulating the fan housing and ducts, both upstream and downstream. Alternately, sound reducing equipment may be installed at the fan inlet or at the fan discharge. For example, U.S. Pat. No. 3,191,851 to Wood describes a two-part system including a square sheet of metal that extends towards and slightly over a small portion of the fan, plus a perforated fairing to decrease size of the fan outlet. U.S. Pat. No. 5,340,275 to Eisinger discloses a rotating cutoff device that is attached within a fan casing. Resonating chambers in the cutoff device are meant to absorb sound. U.S. Pat. No. 6,463,230 to Wargo describes a noise reduction device for smoothing airflow transition at a pinch point of a fan. Wargo focuses on reducing air stagnation at the point where the fan scroll is tangent to the scroll case. The noise reduction device has an airfoil cross section shape, and extends linearly over the fan opening. U.S. Pat. No. 6,575,696 to Lyons et al. combines a sound attenuating cavity, formed as part of the blower housing, with an angled cut off for disrupting pressure fluctuation near the intersection of the exhaust section and the fan scroll.
In another example, U.S. Pat. No. 5,536,140 to Wagner et al. discloses a furnace blower with a flat plate that is inserted parallel to a blower exhaust port. Notches cut in a specified pattern vary the quantity of airflow and reduce pulsing tones. U.S. Pat. No. 5,584,653 to Frank et al. discloses a device for reducing noise in a side channel fan, which appears to include notches or spikes cut into fan outlets and pointing into the intake/discharge, to reduce noise.
U.S. Pat. No. 3,034,702 to Larsson et al. is not concerned with noise suppression, but rather is directed towards a fan having great axial length and dual air inlets, one at each end. Larsson relies upon a series of baffles to provide uniform flow throughout the entire cross-section of the fan discharge opening.
U.S. Pat. No. 6,935,835 to Della Mora discloses various anti-noise stabilizers for centrifugal fans. In particular, Della Mora seeks to homogenize airflow and reduce vortices, in order to reduce noise and improve efficiency of the centrifugal fan. The stabilizers extend for the width of the discharge opening and include dual appendages that face the inlet cone of the fan, one on either side of the discharge opening. U.S. Pat. No. 6,039,532 to McConnell also discloses a device at a fan discharge opening. In particular, McConnell places a baffle in the outlet of a squirrel cage fan. The baffle either tapers continuously from one side of the fan outlet to the other side of the outlet, or is a rectangular insert with a plurality of holes that increase in size from one end to the other end of the baffle.
U.S. Pat. No. 3,687,360 also provides a noise suppressing baffle in a discharge duct. Prew's triangular baffle is inserted within the duct, proximate a chamber housing rotating blades (i.e., a centrifuge chamber). The baffle changes the effective shape of the opening between the duct and the chamber to a trapezoid, and further provides a gradual increase in cross-sectional area of the duct. This change in cross-section decreases velocity of material being discharged into the duct, in order to reduce tendency of the material to build up on walls of the duct.